This invention concerns novel compositions, useful as catalysts, said compositions comprising metals and metal ions, such as ruthenium (Ru) and palladium (Pd), incorporated in an inorganic matrix comprising an inorganic oxide network. Catalyst activity is enhanced versus analogous supported metal catalysts.
E. I. Ko, in the Handbook of Heterogeneous Catalysis, ed. by G. Ertl et al, reviews generally the use of sol-gel processes for the preparation of catalytic materials. There is no disclosure of nor suggestion of ruthenium or rhenium containing catalysts.
U.S. Pat. No. 4,622,310 discloses inorganic phosphate aerogels. The utility stated is as porous inert carrier materials (supports) in polymerization and copolymerization processes. Use of the inorganic phosphates as supports for elements in groups VIB, VIIB and VIII of the Periodic Table is described. There is no disclosure nor suggestion of incorporating the elements within the inorganic phosphate gel matrix.
U.S. Pat. No. 4,469,816 discloses a catalyst composition comprising a uniform dispersion of individual metallic palladium particles impregnated onto, within and throughout an alumina aerogel support processes for the preparation of catalytic materials. There is no disclosure of nor suggestion of ruthenium or rhenium containing catalysts.
U.S. Pat. No. 5,538,931 discloses a process for preparing a supported catalyst comprising a transition metal selected from palladium, platinum, nickel, cobalt or copper on an aerogel support.
DE-A 195 30 528 and DE-A 195 37 202 disclose catalysts comprising ruthenium dispersed in titania and zirconia sol-gel matrices, respectively. No promotors or co-calalysts are described.
This invention provides catalyst precursor compositions comprising catalytic species dispersed in and distributed throughout a high surface area matrix wherein, the catalytic species is selected from the group consisting of ruthenium and palladium, in the optional presence of a promoter selected from the group consisting of rhenium, molybdenum and tin.
The high surface area matrix material may be an inorganic oxide network, optionally prepared by the sol-gel route.
This invention further provides catalyst compositions comprising the reduced form of the above catalyst precursor compositions.
The catalyst precursor composition may further include a promoter.
Preferred promoters are selected from the group consisting of Rhenium, Molybdenum and Tin.
This invention further provides improved processes for the reduction of maleic acid to tetrahydrofuran (THF) and 1,4-butanediol (BDO) and for the reduction of gamma butyrolactone to tetrahydrofuran and 1,4-butanediol, the improvement consisting of the use of the catalysts of the present invention.
The present invention concerns novel catalyst compositions containing metals and metal ions, such as ruthenium (Ru) and palladium (Pd), incorporated into a matrix comprising inorganic oxides and oxyhydroxides of Ti, Nb, Ta, Zr and Si, Al, and others.
As used herein, the term matrix means a skeletal framework of oxides and oxyhydroxides derived from the hydrolysis and condensation of alkoxides and other reagents. The framework typically comprises 30% or more, by weight, of the total catalyst composition. As discussed below, porosity and microstructure can be controlled, in some cases, by synthetic parameters (i.e. pH, temperature), drying, and other heat conditioning. As used herein, the term microstructure means a description, both physical and chemical in nature, of the bonding of domains and crystallites with each other and their arrangement and physical appearance or morphology in a matrix or solid; this term also describes the structure and morphology, that is bonding and physical appearance, of the other active cationic precursors which are included in this invention.
The catalytic species are dispersed in and distributed throughout a high surface area matrix. Alternatively, the catalytic species may be referred to as being xe2x80x9cmatrix incorporatedxe2x80x9d.
Certain promoter materials, for example rhenium (Re), molybdenum (Mo) and tin (Sn), may also be present. Typical preparations involve sol gel chemistry. It is understood that sol gel products can be typically incompletely condensed resulting in products bearing residual hydroxy or alkoxy groups.
The catalyst compositions of the present invention may be prepared by one step synthesis of alcogels in which hydrolyzable matrix precursors are used in the presence of soluble metal salts and promoters. This preparative process is characterized by adding a solution of at least one catalytic metal compound selected from the group consisting of ruthenium and palladium to a solution of at least one metal alkoxide, wherein the metal is selected from the group consisting of Al, Ti, Nb, Zr, Ta, Si and other inorganic alkoxides, and gelling the resulting mixture. The order of addition of reagents, nature of precursors and solvents and the nature of gelling agents may be varied widely. The term gelling agent means a reagent that causes or facilitates the formation of a gel. It may be acidic, basic or neutral, such as of water.
General compositional ranges for the catalyst precursors herein are Ru and Pd from 0.1 to 20 wt %; the promoters Re and Sn from 0 to 20 wt % with the balance being the matrix material.
A typical preparation involves the incorporation of Ru, Pd, Sn, Mo or Re salts, or mixtures thereof, in an alkoxide solution of aluminum, silicon, titanium, zirconium, tantalum or niobium. The hydrolysis of the alkoxides can either be acid or base catalyzed. Hydrolysis of the alkoxide precursors is accompanied by condensation reactions. Under the proper conditions (pH, gelling agent, reactant ratios, temperature, time, solvent and solvent concentration), these can result in the polymerization into an inorganic gel containing the desired catalytic species or precursors. In some cases, the catalytic species are either part of the polymerization network, or are entrapped within the network.
A consequence of this method is that higher metal dispersion and uniformity can be achieved in the inorganic oxide matrix than is normally attainable using more conventional synthetic methods.
The first step in the synthesis of gels consists of preparing solutions of the gel precursors, which may be, but are not limited to, alkoxides and other reagents and separate solutions containing protic solvents such as water. The alkoxide solutions are mixed with the solutions containing the protic solvents, and the alkoxides will react and polymerize to form a gel. The protic solvent can include water, with trace acid or base as catalyst to initiate hydrolysis. As polymerization and crosslinking proceeds, viscosity increases and the material can eventually set to a rigid xe2x80x9cgelxe2x80x9d. The xe2x80x9cgelxe2x80x9d consists of a crosslinked network of the desired material which incorporates the original solvent within its open porous structure. The xe2x80x9cgelxe2x80x9d may then be dried, typically by either simple heating in a flow of dry air to produce an aerogel or the entrapped solvent may be removed by displacement with a supercritical fluid such as liquid CO2 to produce an aerogel, as described below. Final calcination of these dried materials to elevated temperatures ( greater than 200xc2x0 C.) results in products which typically have very porous structures and concomitantly high surface areas.
In the preparation of the catalysts of the present invention, the active metal precursors and promoters can be added to the protic or the alkoxide containing solutions. After gelation, the metal salt or complex is uniformly incorporated into the gel network. The gel may then be dried and heated to produce xerogel or aergoel materials, as described below.
Because of the synthetic technique and the physical appearance of the alcogels materials produced, it is clear that the precursor xerogels and aerogels contain active metals and promoters in a highly dispersed state. Further processing to produce the final catalytic material may include chemical reduction at low temperatures to produce the final highly dispersed material, or a combination of calcination cycles in various media, including hydrogen, to produce the final active catalyst. Activation of the material can be performed on stream, under reaction conditions.
In the practice of this invention one or more inorganic metal alkoxides or salts thereof may be used as starting material for preparing the gels. It is, however, preferred to utilize the metal alkoxides. The rhenium promoter can be added as perrhenic acid in water during the synthesis of the xerogel. It can also be post-added as Re(CO)5Cl to the hydrogenation reaction mixture during the reaction.
The inorganic metal alkoxides used in this invention may include any alkoxide which contains from 1 to 20 carbon atoms and preferably 1 to 5 carbon atoms in the alkoxide group, which are preferably soluble in the liquid reaction medium. In this invention, preferably, C1-C4 systems, ethoxides, n-butoxides or isopropoxides are used.
One of the criteria for the starting material are inorganic alkoxides or metal salts which will dissolve in the specified medium or solvent. Commercially available alkoxides can be used. However, inorganic alkoxides can be prepared by other routes. Some examples include direct reaction of zero valent metals with alcohols in the presence of a catalyst. Many alkoxides can be formed by reaction of metal halides with alcohols. Alkoxy derivatives can be synthesized by the reaction of the alkoxide with alcohol in a ligand interchange reaction. Direct
reaction of dialkylamides with alcohol also forms alkoxide derivatives. The medium utilized in the process generally should be a solvent for the inorganic alkoxide or alkoxides which are utilized and the additional metal reagents and promoters which are added in the single step synthesis. Solubility of all components in their respective media (aqueous and non-aqueous) is preferred to produce highly dispersed materials. By employing soluble reagents in this manner, mixing and dispersion of the active metals and promoter reagents can be near atomic, in fact mirroring their dispersion in their respective solutions. The precursor xerogel thus produced by this process will be highly dispersed active metals and promoters. High dispersion results in precursor particles in the nanometer size range or smaller.
Generally, the amount of solvent used is linked to the alkoxide content. A molar ratio of 26.5 ethanol/total alkoxide is typically used, although a range of 5 to greater than 53 can be used. If a large excess of alcohol is used, gelation will not generally occur immediately; some solvent evaporation is needed. At lower solvent concentrations, it is contemplated that a heavier gel will be formed having less pore volume and surface area.
To prepare the catalysts of the present invention, water and any aqueous solutions are added in a dropwise fashion to the alcohol soluble alkoxide and other reagents, to induce hydrolysis and condensation reactions. Depending on the alkoxide system, a discernible gel point can be reached in minutes or hours. The molar ratio of the total water added (including water present in aqueous solutions), can vary according to the specific inorganic alkoxide being reacted.
Generally, a molar ratio of H2O:alkoxide range of 0.1 to 20 is within the scope of this invention. However, ratios close to 5:1 for tantalum alkoxide, 4:1 for zirconium alkoxide and titanium alkoxides can be used. The amount of water utilized in the reaction can be that calculated to hydrolyze the inorganic alkoxide in the reaction mixture. A ratio lower than that needed to hydrolzye the alkoxide species will result in a partially hydrolyzed material, which in most cases will reach a gel point at a much slower rate, depending on the aging procedure and the presence of atmospheric moisture.
The addition of acidic or basic reagents to the inorganic alkoxide medium can have an effect on the kinetics of the hydrolysis and condensation reactions, and the microstructure of the oxide/hydroxide matrices derived from the alkoxide precursor which entraps or incorporates the soluble metal and promoter reagents. Generally, a pH range of 1-12 can be used, with a pH range of 1-6 preferred for these experiments.
After reacting to form the alcogels of the present invention, it may be necessary to complete the gelation process with some aging of the gel. This aging can range form one minute to over several days. In general, all alcogels were aged at room temperature in air for at least several hours.
The solvent in the gels can be removed in several different ways: conventional drying, freeze and vacuum drying, spray drying, or the solvent can be exchanged under supercritical conditions. Removal by vacuum drying results in the formation of a xerogel. An aerogel of the material can typically be formed by charging in a pressurized system such as an autoclave. The solvent laden gel which is formed in the practice of the invention is placed in an autoclave where it can be contacted with a fluid above its critical temperature and pressure by allowing supercritical fluid to flow through the solvent laden gel, so as to extract the solvent, until the solvent is no longer being extracted by the supercritical fluid. In performing this extraction to produce the aerogel material, various fluids can be utilized at their critical temperature and pressure. For instance, fluorochlorocarbons typified by Freon(copyright) brand fluorochloromethanes and ethanes, ammonia and carbon dioxide are all suitable for this process. Typically, the extraction fluids are fluids which are gases at atmospheric conditions, so that pore collapse due to the capillary forces at the liquid/solid interface are avoided during drying. The resulting material should, in most cases, possess a higher surface area than the non-supercritically dried materials.
The xerogels and areogels thus produced can be described as precursor salts incorporated into an oxide or oxyhydroxide matrix. The hydroxyl content is undefined at this point; a theoretical maximum corresponds to the valence of the central metal atom. Hence, Ta2(O2-x(OH)x)5 possesses a theoretical hydroxyl maximum at x=2. The molar H2O:alkoxide ratio can also affect the final xerogel stoichiometry; in this case, if H2O:Ta less than 5, there will be residual xe2x80x94OR groups in the unaged gel. However, reaction with atmospheric moisture will convert these to the corresponding xe2x80x94OH and xe2x80x94O groups upon continued polymerization and dehydration. Aging, even under inert conditions, can also affect the condensation of the xe2x80x94OH, eliminating H2O, through continuation of crosslinking and polymerization, i.e., gel formation.
The materials of the present invention are useful in hydrogenation reactions. Specific examples include hydrogenation of maleic acid to tetrahydro-furan, butanediol and other products and hydrogenation of gamma butyrolactone into the same or similar products. For the latter case, additional rhenium (preferably used as rhenium carbonyl chloride) may be added to the hydrogenation reactions as co-catalyst. The compositions of the present invention are also useful for the reduction of 3-hydroxypropionaldehyde to 1,3-propane diol.
The experimental results obtained show the novelty and unexpected results of the current invention. In maleic acid hydrogenation, catalytic reactor tests show the titania and zirconia derived aerogel systems containing ruthenium and rhenium are very active for maleic acid hydrogenation to THF. The THF STY (space time yield, mol/hr-kg catalyst) of the matrix incorporated material in ZrO2 shows at least a 3.5xc3x97increase in STY versus supported catalysts. In addition, the maleic acid conversion is increased 10 fold.
The improvement in the single step synthetic method is clearly demonstrated by comparing Example 4, 1 wt % Ru, 4 wt % Re in ZrO2 aerogel with comparative Example 5, 1 wt % Ru, 4 wt % Re on preformed ZrO2 aerogel, presoaked in water prior to impregnation, and comparative Example 6, prepared by impregnation, at incipient wetness, of ruthenium chloride and perrhenic acid on pre-formned ZrO2 aerogel. Catalysts prepared by the process of this invention showed and STY of 44. 1, versus 12.9 and 4.3 for comparative Examples 5 and 6. The  greater than 350% increase in STY is accompanied by a ten-fold increase in % maleic acid conversion, as defined as follows:
conversion=(moles reactant initial-moles reactant final)/(moles
reactant initial)=(moles reactant converted)/(moles reactant initial).
This unexpected improvement is achieved when three components are added in a single step synthesis to produce the gel as compared to individual, sequential additions or impregnations on pre-formed supports, as described in the comparative examples.
The zirconia aerogel prepared by this matrix incorporation method is significantly more active than a comparable catalyst prepared by the more conventional method of supporting soluble Ru and Re on, for example, a preformed ZrO2 aerogel.
A series of aerogels prepared at lower ruthenium loadings (1/3 wt %) show surprising activity for gamma butyrolactone (GBL) hydrogenation, especially when a soluble rhenium complex [Re(CO)5Cl] is added with the aerogel catalyst.
Acid stability tests on titania and tantalum oxide aerogels, and reactor tests on all catalyst systems, have shown that these catalysts are essentially stable towards dissolution in liquid maleic acid.
Typical N2 BET surface areas for these aerogel materials are several hundred m2/g.
Zirconium n-propoxide, titanium n-butoxide, and tantalum and niobium ethoxides produce the highest quality gels when dissolved in ethanol.
In the data presented herein, THF is tetrahydrofuran, BDO is 1,4-butanediol, space time yield (STY) is defined as (mole THF+BDO product/hr-kg catalyst). Selectivity is defined as moles (THF+BDO)/mole (THF+BDO+byproducts). Conversion herein is defined as (moles reactant converted)/(moles reactant initially present).